A remote-control lock system of the type used to unlock the doors or disarm the burglar alarm of a motor vehicle, or to open a garage door or the like basically comprises a transmitter, a receiver, and a mechanical lock. The transmitter emits a radio-frequency, ultrasonic, or even light signal that the receiver is tuned to. When this signal is received, the receiver actuates the lock, either locking or unlocking it, and can also institute other action, such as starting up an actuator for opening or closing a door.
In the simplest systems the transmitter emits a signal in a particular frequency band and the receiver is tuned to this band only. A normally unused band is used to reduce the likelihood of accidental actuation of the lock.
This type of arrangement is particularly susceptible of malfunction, as the particular signal can sometimes be generated accidentally, for instance by so-called dirty portable transceivers or malfunctioning equipment. In addition it is relatively easy for a person vaguely familiar with the lock system to generate a scanning signal which will eventually traverse the band the receiver is tuned to and actuate it. More sophisticated procedures can involve monitoring the location with the lock system in question with a scanning receiver to discover the wave length that is used. The equipment and knowledge to do this is well within the scope of the average industrial spy, burglar, or the like.
As a result recourse is now normally had to coded signals. Such signals are typically numerically coded, normally also in binary fashion as a sequence of short and long pulses, of pulses of two different signals, or of particularly modulated signals. Both the transmitter and receiver are provided with sets of dip switches that can be set by the user at any of the possible codes.
The password procedures for imparting a certain signature or envelope to the signal and creating a so-called code word are well known in the art, as are the systems for receiving, reading, and reacting to it. Using only, for instance, an eight-bit binary-coded signal it is possible to obtain 256 different codings, making accidental generation of the particular code at the particular frequency statistically unlikely. If a six- or eight-digit decimally coded signal is employed, the selection of possible code words becomes vast.
Unfortunately it is still within the ken of a person skilled in electronics and allied fields to clandestinely monitor the coded signal when it is transmitted. Later at the simplest a recording of it can be transmitted to operate the lock, or a transmitter can be constructed to produce the desired code.